1. Field of the Invention
This invention relates to a ring back tone transmission system and more particularly to a ring back tone transmission system of a digital and electronic switching apparatus which transmits a ring back tone informing a calling subscriber of the fact that a called subscriber is now receiving a ringing signal.
2. Description of Prior Art
In a conventional digital switching apparatus, a speech path system is constituted by digital circuits and therefore, it is necessary to provide for each telephone set a subscriber circuit serving a subscriber line interface which handles necessary high voltage signals and analog signals. Accordingly, reduction in size and inexpensive construction of each subscriber's circuit are of matter of importance. For circuit simplification, a continuous ringing signal source producing a ringing signal which drives the telephone bell of a called subscriber set is constructed as a part of a subscriber's circuit and is ON-OFF controlled by a switch, also provided in the subscriber's circuit, for a transmission time and at a period in compliance with intended purpose and application. This leads to difficulties in obtaining synchronization of a ringing signal with a ring back tone, i.e. a signal informing a calling subscriber of the transmission of the ringing signal, although the synchronization has been easy to achieve in a conventional electromagnetic type switching apparatus.
More particularly, in the digital switching apparatus, a plurality of subscriber's circuits are coupled to a digital switching network for connection exchange via digital signal transmission lines, and a ring back tone source is also constituted by a digital circuit.
When a calling subscriber goes off-hook for dialing, a central processing device for controlling the exchange operations carries out a well-known series of operations, including a switching exchange operation, receiving a dial signal, numerical analysis and discrimination of the stored position of the called subscriber. If a called subscriber is idle, then a speech path after acknowledgement by the called subscriber will be selected and pre-blocked. Subsequently, a ringing signal is transmitted to the telephone set of the called subscriber. For the transmission of a ringing signal, the central processing device instructs a terminal processor for controlling the plurality of subscriber's circuits to activate the transmission of a ringing signal. The terminal processor thus instructed ON-OFF controls a switch provided in a subscriber line for a predetermined ringing time at a predetermined period, so that a ringing signal is supplied to the telephone set of the called subscriber.
On the other hand, the central processing device operates to couple a ring back tone source to a calling subscriber's circuit via the switching network in order to provide a ring back tone to a telephone set of the calling subscriber.
The terminal processor is designed so as to control a plurality of subscriber's circuits in common. Therefore, in order to synchronize the timing signal for controlling the application of ringing signal to the called subscriber telephone set with the phase of the ring back tone, it is necessary to provide a device for distributing the phase information of the ring back tone source to ringing signal controllers of the plurality of subscriber's circuits. In addition, in order to control the ringing signal of the called subscriber's circuit in compliance with the phase information, there results an overload on capability of the terminal processor per se and when a plurality of called subscriber telephone sets simultaneously transmit ring back tones, a problem is raised wherein capacity of the ring back tone source and drive power therefor are increased. For these reasons, the synchronization of the ring back tone with the ringing signal becomes costly and conventionally, the ringing signal is made asynchronous with the ring back tone at the cost of convenience.
Moreover, the central processing device must communicate with the calling subscriber terminal processor to exchange an instruction for transmission of the ring back tone and an instruction for deactivation of the ring back tone upon acknowledgement by the called subscriber and its processing capability is overloaded.
Under the circumstances, users accustomed to handle the electromagnetic type switching apparatus in which a synchronized ring back tone and ringing signal are provided are unsatisfied with the existing digital and electronic switching apparatus, and the advent of a digital and electronic switching apparatus having an improved ring back tone transmission has been desired.